1. Field of the Invention
This invention relates generally to electro-mechanical modulators and, in particular, this invention relates to a valve mechanism for an acoustic modulator which modulates air flow passing through the valve at a selected frequency.
2. Description of the Prior Art
In the prior art there are many types of electro-mechanical modulators and choppers which regulate air flow at a selected frequency. One type of prior art electro-mechanical is an acoustic modulator which regulates air flow by creating air pressure variations which creates acoustic energy having a predetermined frequency. This acoustic energy may then be used to conduct noise and vibration testing on both civilian and military aircraft, missiles and other weapons systems.
The functioning part of the acoustic modulator is the valve mechanism. Prior art valve mechanisms generally use a pair of cylindrical shaped members, each member of which has a plurality of ports which when aligned allow pressurized air to pass therethrough. The first cylindrical shaped member is generally stationary and the second member, which is electro-magnetically exited, moves relative to the first member. An electrical sinusoidal input signal is provided to a coil which is positioned within a magnetic field and secured to the second member. This sinusoidal input signal excites the coil causing the ports of the second member to first align with the ports of the first member and then move out of alignment with the ports of the first member. This movement of the second member relative to the first member first increases air flow and then decreases air flow through the valve mechanism of the acoustic modulator which results in the acoustic modulator providing air pressure variations having a frequency equal to the frequency of the sinusoidal input signal.
Unfortunately, this prior art acoustic modulator consumes excessive air at partial modulation levels which is costly, provides excessive background noise and has a frequency response limitation of between 800 and 1000 hertz which is not an optimum desired frequency for vibration testing of military aircraft and the like.
A second prior art acoustic modulator used for testing purposes has a double valve mechanism of the type disclosed in U.S. Pat. No. 4,893,655 which issued Jan. 16, 1990. The double valve mechanism of U.S. Pat. No. 4,893,655 includes a pair of cylindrical shaped members with each cylindrical shaped member having a plurality of elongated ports. Electro-magnetic excitation causes the cylindrical shaped members to move relative to each other which causes the ports of the members to first align opening the valve and then move out of alignment closing the valve, thereby modulating air flow through the acoustic modulator. Linear motion of both cylindrical shaped members of the valve mechanism is only one half that of prior art valve mechanisms resulting in an acoustic output wave front which is twice the frequency of an electrical sinusoidal input signal supplied to the modulator.
While the valve mechanism of U.S. Pat. No. 4,893,655 provides a significant improvement in the frequency of modulation (approximately 2000 hertz) of air flowing through the modulator, this valve mechanism has a complex design, is expensive and is susceptible to failure because of the movement of both cylindrical shaped members of the valve mechanism.
With the disadvantages inherent in the design of prior art valve mechanisms for acoustic modulators, including those mentioned above, the present invention was conceived and one of its objects was to provide a valve mechanism for an acoustic modulator which modulates air flowing through the modulator at a frequency similar to that of the valve mechanism disclosed in U.S. Pat. No. 4,893,655.
Another object of the present invention is to provide a valve mechanism for acoustic modulators which is simplistic in design, highly efficient and still provides for a frequency of modulation approaching that of complex valve mechanisms such as the valve mechanism of U.S. Pat. No. 4,893,655.
These and other objects, advantages and novel features of the present invention will become more apparent from the detailed description of the preferred embodiment of the invention when considered in conjunction with the accompanying drawings.